The air-fuel ratio in a combustion engine affects both engine emissions and performance. With strict modern emissions standards for automobiles, it is necessary to accurately control the air-fuel ratio of the automobile engine, requiring precise measurement of the mass airflow into the engine.
Currently, engine airflow is either measured with a mass airflow sensor or calculated by the speed-density method. Improvements in both types of systems have lead to improved air-fuel ratio control of engines, enabling vehicle manufacturers to meet existing emissions standards. In general, while mass airflow sensors are more accurate than speed-density systems, they are also more expensive.
In an ideal speed-density system, sensor processing and fuel delivery occur instantaneously to allow precise air-fuel ratio control. In reality, however, it takes a finite amount of time to process sensor measurements to compute proper fueling and a finite amount of time to physically deliver the fuel. The delays in the fuel computation and delivery force the fuel control system to compute the fuel to be delivered in a particular cylinder before the actual delivery of the fuel.
In speed-density systems, airflow estimates are based on measures of manifold absolute pressure. The aforementioned delays force speed-density systems to read manifold absolute pressure prior to the theoretically optimal time, which would be during the intake event for the cylinder to be fueled. A typical value for this delay is two to three engine events. Because of the dynamic characteristics of engines, the manifold absolute pressure, and hence airflow, can change dramatically between the time manifold absolute pressure is read (and the fuel computed) and the intake event for the cylinder being fueled. Therefore, in speed-density systems, the lag between the calculated airflow and the actual airflow is prominent. Speed-density calculations are most accurate during static situations. During dynamic situations, when the mass airflow into the engine is changing, the calculated mass airflow into the engine lags the actual mass airflow. This increases the difficulty of properly controlling the air-fuel ratio during transient conditions.
What is desired is a method of achieving increased accuracy in the determination of proper air-fuel ratio for the vehicle engine in vehicles with or without mass airflow meters to enable vehicle manufacturers to meet increasingly tightening emissions standards.